The present invention relates to an electro-optical element that can make lower the coherent characteristic of a light beam, and a method of driving and a method of manufacturing the same.
A laser light has an information accommodation capability because its frequency is generally higher than that of the electric waves. Also, since the wavelength is identical and the phase is arranged properly, the laser light has excellent monochromaticity and directivity. And, the laser light has coherent characteristic (interference-making ability characteristic) that an ordinary light beam does not have. Further, because the laser light can be converged extremely thin, the laser light has the feature of, for example, enabling its energy to be concentrated onto a minute area and thereby enabling a high temperature to be realized locally and instantaneously or the like. For this reason, the laser light has hitherto been applied to many fields such as communication or information relations, measurement relations, applications to the fabrication techniques, utilization in the medical field and so on.
However, since in this way the laser light has not only excellent directivity but also high strength and coherent characteristic, in a case where using a laser light as an illumination light, for example, where using the laser light on a laser display, the dot-like glittering, so-called xe2x80x9cspeckle noisexe2x80x9d; that occurs due to its coherence being high become problematic.
As an optical device for decreasing such speckle noises there is known, for example, a bundle fiber, a rotary diffusion plate, or the like.
The bundle fiber has a structure wherein a number of multi-mode fibers are bundled. The bundle fiber shapes the beam configuration of a coherent light beam so as for it to conform to the configuration of the input end of the bundle fiber. By propagating the light beam through the interior of this bundle fiber, the coherent characteristic thereof is decreased.
Each of the respective optical fibers constituting the bundle fiber is a multi-mode optical fiber. As a result of this, the light beam propagating through the interior of the optical fiber propagates by being separated into a plurality of modes. And, since the respective modes are different in propagation speed from each other, the light beams that have been emitted from the optical fiber become the ones that have a variety of phases, respectively. And, since this optical fiber is bundled large in number, the light beams that have been output from the resulting bundle fiber become the ones having an again larger number of phases. As a result of this, the coherent characteristic is eased with the result that the speckle noises are decreased.
However, the phase relation among the light beams emitted from their corresponding optical fibers is always stable when viewed from the aspect of time. Therefore, it is difficult to completely erase the speckle noises away.
On the other hand, the rotary diffusion plate is constructed so that a coherent light beam may be transmitted through a rotating sheet of frosted glass. First, the rotary diffusion plate is arranged to disturb the relation in phase between the constituting components of a light beam, i.e., between the rays constituting a light beam by the use of the frosted glass in view of space, and then to change the relation in phase between the light rays constituting a light beam by rotating this sheet of frosted glass with a motor, etc. from the aspect of time. Namely, the rotary diffusion plate can be said to be the one that has been improved from the above-described speckle noise decreasing method based on the use of the bundle fiber.
However, in this method, it becomes necessary to provide a movable portion such as a motor or the like for rotating the rotary diffusion plate. Therefore, there is the problem that the device becomes large in size and that the power consumption also becomes large.
The present invention has been made in view of the above-described actual circumstances and has an object to provide an electro-optical element that disturbs the relation in phase between the rays of a light beam having a high coherent characteristic, such as laser light, and that makes it possible to easily obtain a light beam whose coherent characteristic has been decreased, and a method of driving and a method of manufacturing the same.
The electro-optical element according to the present invention is constituted by a single piece of ferroelectric substrate having electro-optical effects, or a laminate of a plurality of similar ferroelectric substrates.
This electro-optical element that is composed of a single, or a plurality of, ferroelectric substrates each have an incident surface and an outgoing surface of a light beam. In the propagation passage for a light beam between the incident surface and the outgoing surface is provided a phase displacement means that is constituted by a plurality of, or a single, polarization inversion domain.
Also, on the above-described electro-optical element are provided at least a pair of electrodes that are used for applying an electric field to the formation region of the polarization inversion domain in the ferroelectric substrate.
The polarization inversion domain constitutes the phase displacement means for making irregularly different from each other the phases of the light rays constituting a light beam that is incident upon the ferroelectric substrate. To this end, the polarization inversion domain is made to have the configurations and/or dispositions that are irregular at least in the propagation direction of the light beam.
Also, it is preferable that the polarization inversion domain constituting the phase displacement means be formed irregularly also in a direction substantially perpendicular to the propagation direction of the light beam.
Further, it is preferable that the polarization inversion domain constituting the phase displacement means have a construction wherein the depth thereof is made irregular.
The incident surface and the outgoing surface of the light beam can each be made a mirror surface, or at least one of them, preferably the outgoing surface, can be made a roughened surface.
Also, a part, or all, of the above-described electrodes can each be made to be a transparent electrode having a high transmittance with respect to the incident light beam, whereby one, or both, of the incidence and the outgo of the light can be made through this transparent electrode.
In the method of driving according to the present invention, in the electro-optical element having each of the above-described constructions, between at least a pair of electrodes thereof is supplied a required electric signal, preferably an A.C. current signal, more preferably an A.C. current signal having D.C. current components superimposed thereon. By doing so, the refractive index of the polarization inversion domain and that of other portions are differentiated from each other. And by this polarization inversion domain that has been irregularly formed, phase displacement is performed with regard to the light rays constituting the light beam that passes through the polarization inversion domain, thereby differentiating the phases of the respective light rays from one another, accordingly, decreasing the coherence, i.e., interference characteristic of the light beam.
In the manufacturing method of the electro-optical element according to the present invention, there is executed a first step for forming, in a part of the ferroelectric substrate constituting the electro-optical element that has each of the above-described respective constructions, a first polarization inversion domain, from one main surface of the substrate, partly, or wholly, in the thickness direction of the ferroelectric substrate, with an irregular pattern.
Next, there is executed a second step for reducing the depth of the first polarization inversion depth to a prescribed depth. Next, there is executed a third step for forming, in another part of the ferroelectric substrate constituting the electro-optical element, a second polarization inversion domain, from one main surface of the substrate, partly, or wholly, in the thickness direction of the ferroelectric substrate.
Further, according to the necessity, similarly, there is executed a fourth step for maintaining the resulting substrate to be at a prescribed temperature not higher than the Curie temperature for a required period of time to thereby reducing the depth of each polarization inversion domain to a required depth.
In the first and the third step, electrodes are provided on both of the mutually opposing main surfaces of the ferroelectric substrate, and a voltage is applied between these electrodes to thereby form the first and the second polarization inversion domain.
Or, in the first and the third step, charged particles having negative or positive charge are radiated onto the surface on the negative side or positive side of the spontaneous polarization, of the ferroelectric substrate to thereby form the first and the second polarization inversion domain.
Also, the second and the fourth step can be executed by maintaining the ferroelectric substrate to be at a prescribed temperature lower than the Curie temperature.
In each of the above-described electro-optical element, the ferroelectric substrate thereof is constituted by a crystal of LiNbx Ta1xe2x88x92x O3 (where 0xe2x89xa6xxe2x89xa61), and there is executed the step for maintaining this substrate to be at a prescribed temperature not higher than the Curie temperature in an atmospheric air or in an atmosphere of oxygen.
Also, in each of the above-described electro-optical element, the ferroelectric substrate thereof is constituted by a crystal of LiNbO3, and there is executed the step for maintaining this ferroelectric substrate to be at a temperature of from 300xc2x0 C. to 1150xc2x0 C. within 30 hours as counted from the instantaneous moment in an atmospheric air or in an atmosphere of oxygen.